Both human and mouse bone marrow, when cultured in soft agar, form colonies of granulocytic and monocytic cells under the influence of colony stimulating factor (CSF) from both mouse and human sources. High levels of CSF are obtained by administration of endotoxin to the mouse. It is proposed in this study to evaluate the mechanism of CSF production by endotoxin, as well as the action of CSF in the bone marrow of the mouse, and to follow this by a study of action of CSF in a disease characterized by a progressive replacement of the bone marrow by neoplastic cells (mouse lymphoma). Since human peripheral blood cells can release CSF after incubation in vitro (conditioned medium) and this CSF stimulates colonies in mouse bone marrow, we also propose to study the effect of endotoxin on the CSF release by human leukocytes in health and disease.